Carbon-13 (.sup.13 C) is a naturally occurring, non-radioactive isotope of Carbon, and .sup.13 C makes up about 1% of all Carbon on Earth, with most of the remaining 99% of Carbon being composed of Carbon-12(.sup.12 C). Measurements of the variations in the ratio of .sup.13 C/.sup.12 C in substances can provide valuable information to biologists, geologists, and medical personnel because various processes affect the value of the ratio of .sup.13 C/.sup.12 C in known ways.
As an example of one application wherein knowledge of the ratio of .sup.13 C/.sup.12 C can be important, Carbon found in soil which includes organic matter from plant life has a relatively low .sup.13 C/.sup.12 C ratio, compared to Carbon from inorganic sources. Consequently, it might be inferred whether a planet such as Mars has ever supported Carbon-based plant life by analyzing the .sup.13 C/.sup.12 C ratio in various Martian soil samples.
As another example, human disease can be diagnosed by measuring the ratio .sup.13 C/.sup.12 C in a patient's breath. Diagnosing disease by analyzing the .sup.13 C content of breath affords many important advantages over other diagnostic tests, including safety, painlessness, quick results, and comparatively low cost.
To diagnose disease based upon .sup.13 C analysis, a patient first ingests a capsule containing a food-like substance, referred to as a "substrate", which has been specifically isotopically enriched with .sup.13 C. Because different substrates are digested by different enzymes, it is possible to target particular organs for testing by tailoring the substrate to that organ. Indeed; many diseases may be diagnosed using such breath analysis, and a variety of substrates have been proposed or developed for medical diagnosis. Currently, these substrates include .sup.13 C urea test for Helicobacter pylori bacteria in the gastrointestinal tract (a cause of ulcers and gastritis), .sup.13 C galactose test for liver function and cirrhosis, .sup.13 C-Hiolein.RTM. test for fat malabsorption, .sup.13 C-Neolate.RTM. test for pancreatic and small intestine mucosal dysfunction, .sup.13 C xylose test for small intestine bacterial overgrowth, .sup.13 C glucose test for carbohydrate malabsorption, .sup.13 C starch test for pancreatic amylase, .sup.13 C protein test for protein metabolism, .sup.13 C-amino acids for inborn errors of metabolism, and .sup.13 C lactose test for lactose intolerance. It will be appreciated that many more such tests with associated substrates may be developed.
Once ingested, the substrate is digested or otherwise processed by the targeted organ, and Carbon Dioxide (CO.sub.2) resulting from the digestion is transported by the patient's venous blood to the patient's lungs, where the CO.sub.2 is transferred through the alveolar walls of the lungs and exhaled. The ratio of .sup.13 CO.sub.2 to .sup.12 CO.sub.2 in the patient's breath is then measured by a special purpose instrument. Normal individuals will show a different ratio than individuals having the disease, thereby providing a diagnostic tool to the physician. Importantly, the underlying analytical technique-measuring the ratio of .sup.13 CO.sub.2 to .sup.12 CO.sub.2 in the patient's breath, remains the same for all the diseases which can be diagnosed in this fashion. Therefore, a single purpose instrument can perform a wide variety of tests, when used with different substrates.
Breath tests for diagnosing disease, however, are not yet in general use, primarily because of the high cost of instruments that are currently used for isotopic analysis. The most common such instruments are mass spectrometers, which are elaborate instruments with unit costs in the $150,000 to $500,000 range. Also, mass spectrometers typically require skilled technicians to operate. This makes breath testing economically impractical for widespread use in doctors' offices or hospitals.
Alternative analysis instruments which are less expensive than mass spectrometers accordingly have been introduced for measuring the ratio of .sup.13 C/.sup.12 C in a patient's breath. More specifically, instruments have been introduced that employ the principles of optical spectroscopy to measure the ratio Of .sup.13 C/.sup.12 C in a patient's breath. Optical spectroscopy is a method of measurement in which gaseous materials can be detected and measured by their ability to absorb optical energy at certain wavelengths. Each gaseous material has characteristic wavelengths at which it absorbs light energy. When multi-spectral light covering a range of wavelengths is transmitted from a light source through a gas, the composition of the gas is indicated by the specific wavelengths absorbed and the magnitude of the absorption.
Lasers are widely used as the light source in optical spectroscopy, owing to the narrow spectral emission line afforded by the use of coherent light. Of particular importance to the present invention are diode laser light sources. A diode laser is a class of laser based on semiconductor technology. It consists of several layers of different materials on a crystalline substrate, such as silicon, gallium arsenide, or lead sulfide. Diode lasers have the property of emitting light in a very narrow wavelength band when electrical current is applied to them.
An example of one such device is disclosed in U.S. Pat. No. Re. 33,493 to Lee et at. for a diode laser gas analysis spectrometer that operates at a wavelength of 3-6 .mu.m. Because the primary (i.e., .nu..sub.3 fundamental) absorption band for Carbon Dioxide is approximately 4.3 .mu.m, the Lee et al. apparatus operates at an optimal wavelength for purposes of detecting the ratio of C.sup.13 /C.sup.12. Unfortunately, the Lee et al. apparatus requires cryogenic cooling, rendering it inappropriate both for spacecraft uses and medical diagnostic uses.
An alternate diode laser-based spectrometer is disclosed in U.S. Pat. No. 5,317,156 to Cooper et al., which uses a diode laser made of indium-gallium-arsenide-phosphorus (InGaAsP) that can operate at room temperature and consequently does not require cryogenic cooling. Unfortunately, the Cooper et al. spectrometer has an operating wavelength of only 1.6 .mu.m, which means that the absorption spectra it can detect is at a weaker band of Carbon Dioxide absorption, instead of the .nu..sub.3 fundamental absorption band for Carbon Dioxide. Consequently, because the amount of absorption by Carbon Dioxide in the 1.6 .mu.m region is orders of magnitude less than the amount of the absorption in the .nu..sub.3 fundamental region, the Cooper et al. apparatus intrinsically suffers degraded sensitivity. As applied to breath analysis, sensitivity of measurement is an important parameter in a diode laser spectrometer, because using a less-sensitive instrument requires administering a greater dosage of .sup.13 C-bearing substrate to a patient, increasing costs and potentially complicating regulatory approval.
Accordingly, it is an object of the present invention to provide a spectrometer for analyzing the ratio of .sup.13 C/.sup.12 C in a substance which operates at the 4.3 .mu.m absorption wavelength region for Carbon Dioxide. Another object of the present invention is to provide a spectrometer for analyzing the ratio of .sup.13 C/.sup.12 C in a substance which does not require cryogenic cooling. Still another object of the present invention is to provide a spectrometer for analyzing the ratio of .sup.13 C/.sup.12 C in a substance which is easy to use and cost effective.